[Bug] /ultraplan fails with git authentication despite valid GitHub credentials

Bug Description
/ultraplan creates a session on claude.ai/code but fails with
"Authentication failed while accessing the git_repository source"
every time.

However, creating a session manually on claude.ai/code with the
same repo works perfectly — clone, branches, everything is visible.

Steps to reproduce:

1. gh auth status shows correct scopes (repo, read:org, admin:public_key)
2. Claude GitHub App installed with "All repositories" access
3. /web-setup completes successfully, opens draft session
4. Draft session shows "No branches available" and no repo selected
5. /ultraplan <any prompt> → new session → authentication error
6. Manual session on claude.ai/code with same repo → works fine

Expected: /ultraplan should use the same credentials path as manual
claude.ai/code sessions.

Account: Max 20x
Repo: private GitHub repo
OS: macOS

Environment Info

• Platform: darwin
• Terminal: Apple_Terminal
• Version: 2.1.97
• Feedback ID: b53124d4-0086-4dbe-a3a9-408ba2e09b38

Errors

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